Continuous Process for the Preparation of Phenol from Benzene in a Fixed Bed Reactor

ABSTRACT

The invention relates to a continuous process for the preparation of phenol by means of the direct oxidation of benzene with hydrogen peroxide in the presence of a catalyst based on titanium silicalite TS-1 comprising: (a) running the process in a fixed bed reactor containing the catalyst based on TS-1 at a temperature ranging from 80-120° C. and at a pressure ranging from 3-15 atm; (b) feeding to the reactor a stream containing H 2 O 2 , benzene, sulfolane and water in a single or double phase, wherein the quantities of the single components are within the range of 0.2-6, 15-60, 30-80, 0.5-30 parts by weight, respectively, for every 100 units fed and whose total flow rate is calculated so that the residence time in the reactor (defined as the ratio between the quantity of catalyst by weight and the feeding flow rate) ranges from 0.3 to 2 min; (c) recovery of the products, by-products and solvent from the liquid stream leaving the reactor.

The invention relates to a continuous process for the preparation ofphenol by the direct oxidation of benzene with hydrogen peroxide,carried out in a fixed bed reactor.

More specifically, the invention relates to a process for thepreparation of phenol wherein the oxidation reaction is carried out inthe presence of a catalyst based on titanium silicalite and underparticular operative conditions.

The invention also relates to a process for the activation of thecatalyst based on titanium silicalite TS-1.

Phenol is an industrial intermediate of great importance, which iswidely used in the production of polycarbonates or other phenolicresins.

Phenol is currently produced according to the “Hock process”, whichcontemplates the alkylation of benzene to cumene and subsequentoxidation of cumene to hydroperoxide, which decomposes to phenol andacetone.

Various processes are known in the art for the preparation of phenol,which are based on the direct oxidation of benzene with hydrogenperoxide, in the presence of suitable catalytic systems.

A process is known, for example, which is carried out in the presence ofa catalyst based on titanium silicalite and in an organic solventcapable of enhancing the contact between the organic substrate andhydrogen peroxide.

The conversion and selectivity of processes for the preparation ofphenol by direct oxidation can be improved by operating in the presenceof specific solvents, such as sulfolane, for example (EP A 919531).

In this case, the process is carried out in a batch-type reactoroperating in a two-phase reaction system consisting of the solidcatalyst and an organic phase comprising sulfolane/water/benzene in sucha ratio as to make the reaction mixture homogeneous.

Improvements in the productivity of processes for the production ofphenol can also be obtained by activation of the catalyst with hydrogenperoxide and fluorine ions, as described in European patent applicationEP A 958861.

Further improvements can be obtained by operating in a three-phasereaction system consisting of a solid catalyst, an aqueous phase and anorganic phase comprising the aromatic compound and the solvent, asdescribed in patent application PCT/EP02/12169.

The processes for the preparation of phenol by the direct oxidation ofbenzene described in the known art are generally carried out inCSTR-type reactors in which the catalyst based on titanium silicalite iskept in suspension in the form of a fine powder.

In these processes, the separation of the reaction effluent from thecatalyst can only be effectively carried out by filtration inside thereactor.

This operation, however, cannot be effected in the processes of theknown art because, as a result of the low concentrations of theproducts, the filtrating surface necessary would be too high.

The only possible technological solution consequently consists ineffecting the filtration outside the CSTR reactors. This, however, istechnologically complex due to the necessity of using both additionalequipment and abrasion-resistant materials and also of maintaining theefficiency of the filters.

A process has now been found which allows the direct synthesis of phenolfrom benzene with high conversions and productivities and at the sametime eliminating the problems due to filtration.

In practice, the invention envisages that the process be carried out incontinuous in a fixed bed reactor, operating under particular operatingconditions and according to two different procedures: in homogeneousliquid phase or with two liquid phases using, in both cases, sulfolaneas solvent.

In accordance with this, the object of the present invention relates toa continuous process for the preparation of phenol by means of thedirect oxidation of benzene with hydrogen peroxide in the presence of acatalyst based on titanium silicalite TS-1 comprising:

-   (a) running the process in a fixed bed reactor containing the    catalyst based on TS-1 at a temperature ranging from 80-120° C. and    at a pressure ranging from 3-15 atm;-   (b) feeding to the reactor a stream containing H₂O₂, benzene,    sulfolane and water in a single or double phase, wherein the    quantities of the single components are within the range of 0.2-6,    15-60, 30-80, 0.5-30 parts by weight, respectively, for every 100    units fed and whose total flow rate is calculated so that the    residence time in the reactor ranges from 0.3 to 2 min (wherein the    residence time is the ratio between the weight quantity of catalyst    and the feeding flow rate);-   (c) recovery of the products, by-products and solvent from the    liquid stream leaving the reactor.

The process of the present invention allows different advantages to beobtained:

-   -   it considerably simplifies the operations involved in oxidation        processes of aromatic substrates, as filtration is no longer        required and it also avoids recycling of part of the liquid        reaction effluent;    -   it reduces the investment costs for the production of the plant;    -   it makes the process more versatile as it is also possible to        operate with two liquid phases (three-phase system).

The process of the invention can be effectively carried out incontinuous and operating with several reactors.

In this case, it has been found that it is possible to improve theselectivity, by subdividing the feeding of hydrogen peroxide in equalparts between the various reactors.

The process of the invention is carried out in a fixed bed reactor(plug-flow) charged with the catalyst based on titanium silicalite.

The catalysts of the invention are titanium silicalites with an MFIstructure and having the general formula xTiO₂.(1−x)SiO₂ with x rangingfrom 0.0001 and 0.04.

The structural characteristics of these catalysts as also theirpreparation method are described in the patents U.S. Pat. No. 4,410,501,U.S. Pat. No. 4,954,653, U.S. Pat. No. 4,701,428, EP A906784.

The catalysts of the invention are used in extruded form by formulatingthe titanium silicalite with an inert ligand, such as silica, accordingto the techniques known to experts in the field.

Examples of these formulations can be found in patents U.S. Pat. No.6,491,861, U.S. Pat. No. 6,551,546, US 2003/0078160, US 2003/0130116.

The reaction for the production of phenol by the direct oxidation ofbenzene with H₂O₂, can be carried out according to two differentprocedures: in homogeneous liquid phase (double phase system) or withtwo liquid phases (triple phase system) in both cases using sulfolane assolvent.

In the process for the production of phenol, the feeding ratio betweenH₂O₂, benzene, sulfolane and water lead to the presence of a singleliquid phase or two immiscible phases.

When operating in a single liquid phase, the feeding quantities of H₂O₂,benzene, sulfolane and water generally range from 0.2-4, 20-60, 40-80,0.5-5 parts by weight, respectively, per 100 units fed; whereas whenoperating in the presence of two immiscible phases, the quantities rangefrom 0.3-6, 15-50, 30-70, 10-30.

In both cases, the total flow-rate is calculated so that the residencetime ranges from 0.3 to 2 min.

The oxidation of benzene to phenol with hydrogen peroxide is preferablyeffected in a system consisting of a series of fixed bed plug-flowreactors (from a minimum of two to a maximum of 10 reactors).

Each reactor consists of a single catalyst bed with or without an outerheating jacket.

In this latter case, the reactors operate adiabaticcally and theactivation treatment of the catalyst is effected in a specificapparatus.

The scheme of the reaction section shown in FIG. 1 considers, inparticular, six tubular reactors called PLR1, . . . , PLR6. The liquidflows indicated are intended as the homogeneous liquid phase or the sumof the two immiscible phases, according to the cases.

All the benzene and sulfolane necessary for the reaction are mixed(MIX110) and sent to the first reactor (R110).

The hydrogen peroxide in aqueous solution, on the other hand, issubdivided into equal parts and added (in MIX110-MIX160) to the mainflow before each reactor in order to keep the concentration low andconsequently limit its decomposition to oxygen.

Each reactor completely uses up the quantity of hydrogen peroxide fed.

By subdividing the hydrogen peroxide among various reactors, theconcentration of phenol formed in the first reactors is also kept low;in this way, it is possible to limit reactions leading to the formationof by-products (catechol and hydroquinone) more effectively than whenoperating with a lower number of reactors.

The quantity of reagents fed to the various reactors varies according towhether the procedure is effected in single or double phase.

Typically, when operating in single phase, the quantities of thecomponents of the feeding stream (expressed in parts by weight per 100units fed) range from 0.2-4 for the hydrogen peroxide, 20-60 for thebenzene, 40-80 for the sulfolane, 0.5-5 for the water and preferably0.2-4 for the hydrogen peroxide, 30-50 for the benzene, 45-70 for thesulfolane, 0.5-5 for the water.

Operating in double phase, on the other hand, the quantities of thecomponents of the feeding stream (expressed in parts by weight per 100units fed) range from 0.3-6 for the hydrogen peroxide, 15-50 for thebenzene, 30-70 for the sulfolane, 10-30 for the water and preferably0.3-5 for the hydrogen peroxide, 20-40 for the benzene, 35-60 for thesulfolane, 10-30 for the water.

Nitrogen is sent to the top of each reactor to keep the vapours outsidethe explosive limit.

The reactors can operate either under adiabatic conditions or in thepresence of thermostat-regulation, with a temperature range of 80-120°C. and a pressure of 3-15 atm, preferably 90-110° C. and 5-7 atm.

A heat exchanger (MIX110, . . . , MIX160) is present between one reactorand another to lower the temperature of the outgoing flow before feedingit to the subsequent reactor.

In the case of adiabatic reactors, the absence of thermal exchange (inthe reactors) makes them particularly simple from the point of view ofconstruction.

The total feeding flow-rate is calculated so that the residence time inthe reactor ranges from 0.3 to 2 min.

By operating in coherence with the invention described so far, 100%conversions of H₂O₂ are obtained, with a selectivity of benzene in theorder of 85-92%.

Furthermore, by operating in double phase, the reaction rate isincreased with respect to the homogeneous phase, which allows lowerquantities of catalyst and smaller reactors to be used and also limitingthe temperature to lower values.

The double phase allows the selectivity of benzene to be kept unalteredas also the quantity of catechol and hydroquinone formed.

The catalytic properties of the catalyst used in the oxidation ofbenzene can be improved by means of an activation treatment withfluorides and hydrogen peroxide.

The treatment can be effected on the powder, before extrusion, in abatch process, the treatment being followed by a series of washings withwater and filtrations (as described in patent EP 958861).

It has now been found that the activation process can be carried out incontinuous directly in the column in which the synthesis reaction takesplace: in this way, the treatment is simpler and more easy to apply onan industrial scale.

The activation process of the present invention envisages that thequantities of catalyst to be treated and reagents be selected so as tohave a fluorine/titanium molar ratio ranging from 0.5 to 3.0, preferably2.5 and an H₂O₂/titanium molar ratio ranging from 3.0 to 15, preferably11.

The catalyst can be used as such or closely mixed with an inertmaterial, of similar dimensions, in a quantity equal to the weight ofthe catalyst itself.

The inert material can be selected from quartz, corundum, ceramicmaterial, glass, extruded silica, preferably quartz.

The catalyst or its mixture with the inert material is charged into thereactor which is subsequently brought to temperatures ranging from 20 to120° C., preferably 80° C., and fed with an aqueous solution of ammoniumacid fluoride (NH₄HF₂) and hydrogen peroxide.

The feeding time ranges from 2 to 6 hours, preferably 4 hours.

The feeding solution contains ammonium acid fluoride in a concentrationranging from 0.1% to 1% by weight, preferably 0.25%, and hydrogenperoxide in a concentration ranging from 3% to 10% by weight, preferably4.8%. At the end of the reaction, the reactor is fed with water toeliminate the reagent residues and then emptied of the liquid and thecatalyst undergoes a calcination or drying treatment.

The liquid effluent leaving the reactor at the end of the reactioncontaining the products and by-products as well as the solvent to berecovered for re-use, must be subjected to the purification section.

The purification of the products and recovery of the solvent can beeffected by means of the process described in patent EP 3076502.

This process, however, has the disadvantage that the solution comingfrom the reactor and sent to the distillation section contains a highquantity of water and sulfolane whose presence makes the production ofpure phenol difficult.

A process has now been found for the recovery of the products andsolvent which allows the quantity of water and sulfolane sent todistillation to be reduced, consequently obtaining more efficient andeconomical distillation operations.

In practice, the process of the invention envisages that the two-phaseliquid stream leaving the reactors be subjected to a liquid-liquidextraction using water and benzene as extraction solvents.

The water is fed to the head of the column and has the function ofprevalently extracting the sulfolane, whereas the benzene, which is fedto the bottom of the column, has the function of prevalently extractingthe phenol.

The biphenols and other by-products are divided into the two streams.

The organic stream, enriched in benzene and phenol is sent todistillation whereas the aqueous stream, enriched in water and sulfolaneis sent directly for salification to recover the biphenols.

More specifically, the reaction mixture, containing benzene, water,phenol, sulfolane and reaction by-products (biphenols) , is sent to oneor more extraction columns to which benzene and water are also sent. Thewater (fresh or recycled from other equipment) is fed from above, thereaction mixture is fed to an intermediate point, whereas the benzene(fresh or recycled from other equipment) is sent from below. A lightorganic phase, richer in benzene and phenol and with less sulfolane andwater with respect to the mixture coming from the reaction, leaves thehead of the column; a heavy aqueous phase richer in water and sulfolaneand poorer in benzene and phenol with respect to the mixture coming fromthe reaction leaves the bottom of the column.

The organic phase is sent to the distillation section, whereas theaqueous phase is sent to the salification unit of biphenols.

The distillation and biphenol recovery sections have a configurationwhich is analogous to that described in U.S. patent Ser. No. 10/716,460,but thanks to the previous extraction section, have reduced dimensionsand lower energy consumptions.

In addition to the separation and purification of the phenol, theprocedure adopted allows the purified solvent containing the necessarybenzene for recycling to the oxidation reactor to be obtained, as wellas biphenols dissolved in water which are then retransformed into phenolby means of catalytic hydrodeoxygenation, as described in U.S. Ser. No.10/716,460).

EXAMPLE 1 Activation of the Catalyst

7100 kg of TS1 catalyst (extruded with 15% by weight of silica) arecharged into a tubular reactor, as described in Examples 2 and 3, whichis brought to a temperature of 80° C.

207 kg of ammonium acid fluoride (NH₄HF₂) and 3944 kg of hydrogenperoxide at 30% by weight, are dissolved in 81,000 kg of distilledwater, and fed to the reactor with a flow-rate of 20 m³/h. At the end ofthe addition, the reactor is fed with 40,500 kg of water to remove thereagent residues. Finally, the reactor is emptied and the temperature isbrought to 350° C. for 8 hours, under a flow of inert gas.

The catalyst thus treated is ready to be used for the synthesis ofphenol, and proves to have the following composition: SiO₂=98.20%,TiO₂=1.80%.

EXAMPLE 2 Preparation of Phenol in Homogeneous Phase

The benzene and sulfolane fed to MIX110 are 251.1 t/h and 363.35 t/hrespectively. Each of the six streams of hydrogen peroxide (at 40% byweight of water) to 3.36 t/h.

MIX110 mixes and E110 heats the feeds from room temperature to 100° C.

In R110 the hydrogen peroxide is completely converted to reactionproducts and, by operating adiabaticcally, the temperature rises from100° C. to 112° C.

MIX120 mixes the effluent of the first reactor with the second quantityof hydrogen peroxide and E120 cools this stream to 100° C. As thequantities of hydrogen peroxide are comparable, the temperature increasein R120 also passes from 100° C. to 112° C.

The subsequent MIX130, . . . , MIX160 and E130, . . . , E160 have thesame function as MIX120 and E120 and the reactors R130, . . . R160behave analogously to R120.

Each of the reactors contains 9,000 kg of catalyst (activated asdescribed in example 1), and has the following dimensions: D=2.75 m,H=3.14 m.

The liquid effluent leaving the last reactor has a flow-rate of 639.8t/h and the following composition: benzene=35.9% w, phenol=3.20% w,catechol=0.55% w, hydroquinone=0.29% w, sulfolane=56.79% w, water=3.23%w.

The overall performances obtained are: Conversion of H₂O₂ [%] 100Selectivity of benzene [%] 87.5 Selectivity of H₂O₂ [%] 74.0 H₂O₂ to O₂[%] 5.3 H₂O₂ to (catechol + hydroquinone) [%] 21.0

EXAMPLE 3 Preparation of Phenol in Double Phase

The benzene, water and sulfolane fed to MIX110 are equal to 143.19 t/h,2,226.33 t/h and 96.36 t/h respectively. Each of the six streams ofhydrogen peroxide (at 65% by weight of water) is equal to 6.12 t/h.

MIX110 mixes and E110 heats the feeds from room temperature to 95° C.

In R110 the hydrogen peroxide is completely converted to reactionproducts and, by operating adiabatically, the temperature rises from 95°C. to 108.5° C.

MIX120 mixes the effluent of the first reactor (bi-phasic mixture) withthe second quantity of hydrogen peroxide and E120 cools this stream to95° C. As the quantities of hydrogen peroxide are comparable, thetemperature increase in R120 also passes from 95° C. to 108.5° C.

The subsequent MIX130, . . . , MIX160 and E130, . . . , E160 have thesame function as MIX120 and E120 and the reactors R130, . . . , R160behave analogously to R120.

Each of the reactors contains 7,100 kg of catalyst (activated asdescribed in example 1), and has the following dimensions: D=2.54 m,H=2.91 m.

The liquid stream (biphasic) leaving the last reactor, has a flow-rateof 493.80 t/h and the following composition: benzene=23.73% w,phenol=4.41% w, catechol=0.54% w, hydroquinone=0.29% w, sulfolane=45.83%w, water=24.73% w.

The overall performances obtained are: Conversion of H₂O [%] 100Selectivity of benzene [%] 85 Selectivity of H₂O₂ [%] 61.5 H₂O₂ to O₂[%] 16 H₂O₂ to (catechol + hydroquinone) [%] 19.8

1. A continuous process for the preparation of phenol by the directoxidation of benzene with hydrogen peroxide in the presence of acatalyst based on titanium silicalite TS-1 comprising: (a) running theprocess in a fixed bed reactor containing the catalyst based on TS-1 ata temperature ranging from 80-120° C. and at a pressure ranging from3-15 atm; (b) feeding to the reactor a stream containing H₂O₂, benzene,sulfolane and water in a single or double phase, wherein the quantitiesof the single components are within the range of 0.2-6, 15-60, 30-80,0.5-30 parts by weight, respectively, for every 100 units fed and whosetotal flow rate is calculated so that the residence time in the reactorranges from 0.3 to 2 min; (c) recovery of the products, by-products andsolvent from the liquid stream leaving the reactor.
 2. The processaccording to claim 1, wherein the process is effected at a temperatureranging from 90-110° C. and a pressure ranging from 5-7 atm.
 3. Theprocess according to claim 1, wherein the stream containing H₂O₂,benzene, sulfolane and water is in double phase and the quantities ofthe single components respectively range from 0.3-6, 15-50, 30-70, 10-30parts by weight per 100 units fed and whose total flow-rate iscalculated so that the residence time in the reactor ranges from 0.3 to2 min.
 4. The process according to claim 3, wherein the quantities ofthe single components respectively range from 0.3-5 20-40, 35-60, 10-30parts by weight per 100 units fed.
 5. The process according to claim 1,wherein the oxidation of the benzene is effected in a system consistingof from two to ten fixed bed plug-flow reactors arranged in series. 6.The process according to claim 5, wherein the feeding of the hydrogenperoxide is subdivided into equal parts between the reactors, and isadded to the feeding stream.
 7. The process according to claim 5,wherein the residence time of the feeding stream in each reactor rangesfrom 0.4 to 2.0 minutes.
 8. The process according to claim 1, whereinthe catalyst is used in extruded form.
 9. A process for the activationof the catalyst based on titanium silicalite TS-1 of claim 1,comprising: feeding to the reactor containing the catalyst as such ormixed with an inert material, at a temperature ranging from 20 to 120°C., an aqueous solution containing ammonium acid fluoride in aconcentration ranging from 0.1% to 1% by weight and hydrogen peroxide ina concentration ranging from 3% to 10% by weight, for a time rangingfrom 2 to 6 hours; feeding water to the reactor at the end of thereaction; drying or calcining the catalyst contained in the reactor. 10.The process according to claim 9, wherein the reactor is fed, at atemperature of 80° C., with an aqueous solution containing ammonium acidfluoride at a concentration of 0.25% and hydrogen peroxide at aconcentration of 4.8%.
 11. The process according to claim 9, wherein theaqueous solution containing ammonium acid fluoride and hydrogen peroxideis fed in such a quantity that the molar ratio between the fluorinecontained in the solution and the titanium contained in the catalystranges from 0.5 to 3.0, and the molar ratio between the H₂O₂ and thetitanium ranges from 3.0 to
 15. 12. The process according to claim 11,wherein the molar ratio between the fluorine and titanium is 2.5. 13.The process according to claim 9, wherein the inert material is selectedfrom quartz, corundum, ceramic material, glass, extruded silica.
 14. Theprocess according to claim 13, wherein the inert material is quartz. 15.The process according to claim 1, wherein the catalyst based on TS-1 isactivated according to the process of claim
 9. 16. The process accordingto claim 1, wherein the recovery of the products, by-products andsolvent from the liquid stream leaving the reactor, comprises: sendingthe liquid stream leaving the reactors containing benzene, water,phenol, sulfolane and the reaction by-products to an intermediate pointof one or more extraction columns; feeding fresh or recycled water fromthe top of the extraction column; feeding fresh or recycled benzene fromthe bottom of the distillation column; sending the light organic phaseleaving the head of the distillation column, richer in benzene andphenol and poorer in sulfolane and water, to the distillation section;sending the heavy aqueous phase richer in water and sulfolane and poorerin benzene and phenol, to the salification section of biphenols fortheir recovery; recycling the purified sulfolane containing benzene,recovered in the distillation section, to the oxidation reactor.